It's probably flagged by people who want to save the author from himself. Seriously, he sounds like a stalker in that post and would be well advised to take it down asap.

Users flagged it. That's nearly always the answer to this perennial question.

In fact it would be flagkilled (i.e. marked [flagged] and closed to new comments) several times over, except the software doesn't do that when the thread has many comments.

It probably wasn't mods, it doesn't take a lot of users flagging an article before it falls off the front page. It's also got a fairly high comment / point ratio, which also causes it to fall faster.

I went through over 300 of the top stories and couldn't find it. It was still in its expected position (106) in the "new" pages.